1. Field of the Invention
The present invention relates to an image reading apparatus.
2. Description of the Related Art
Conventionally, this type of image reading apparatus moves a scanner unit for scanning an original optically to perform a home position search for determining an initial position before an image reading operation and then moves the scanner unit to an original reading start position or performs a shading compensation for correcting an output of an optical sensor. Then, importance has been attached to how these operations should be completed fast to reduce a time period between an input of an image reading instruction and completion of an image data output (hereinafter, referred to as FCOT).
For example, the scanner unit is moved to a home position on opening or closing a pressure plate of an original reading apparatus to perform a shading compensation or the scanner unit is moved to the home position when an original is placed on an original feeder. In addition, Japanese Patent Application Laid-open No. 5-14609 discloses a plurality of home positions and changes a home position according to a magnification for reading, thereby reducing FCOT slightly.
Conventionally, however, a position setting for a home position or a shading compensation was always needed before reading an original and it was hard to reduce the time for the position setting or the shading compensation.